Flash memory is a common memory, which has been widely used in various kinds of digital storage devices. The flash memory usually stores information using a floating gate structure. Various kinds of flash memories are developed according to the difference of the floating structure. NOR flash memory is an important kind of flash memory. Intel has introduced ETOX™ NOR flash memory in the last century, the NOR flash memory has a large market share in the memory market.
FIG. 1 is a schematic view of a conventional NOR flash memory. Referring to FIG. 1, two NOR flash memory units, i.e. 10a and 10b of the NOR flash memory, are symmetric to each other and share a source (S), and the flash memory units 10a and 10b are symmetric. A metal silicide layer 130 is located on a floating gate, and can be used to bias a gate signal on the floating gate. 160 is a dielectric layer at least surrounding the floating gate, 140 is an ILD (interlayer dielectric) barrier layer, 150 is a drain electrode (it is usually a drain contacting hole) led out by the drain (D) of the NOR flash memory unit.
In an erasing operation of the NOR flash memory, electrons in the floating gate are erased through an F-N tunneling effect. Referring to FIG. 1, during erasing process, the metal silicide layer 130 on the floating gate is biased a 16 V or larger voltage, the drain electrode 150 is floating. The drain electrode 150 is electrical isolated from the gate electrode through the dielectric layer 160 and/or the ILD barrier layer 140 (the actual thicknesses of the dielectric layer 160 and the ILD layer 140 are much smaller than that shown in FIG. 1). When the voltage difference between the gate electrode and the drain electrode 150 is very large, a drain current shown in FIG. 1 will be induced.
The NOR flash memory continuously scales down, for example, as the critical dimension of the NOR flash memory is smaller than 0.13 μm, an isolation layer (such as the dielectric layer 160 and the ILD barrier layer 140) between the drain electrode 150 and the gate electrode become much thinner, a drain current problem during the erasing process of the gate electrode and the drain electrode 150 become much worse.